Optical coherence tomography (OCT) and speckle imaging are two coherence-based imaging modalities with burgeoning applications in rapid non-invasive measurement of the skin. Both modalities are sensitive to tissue morphology and have polarization-sensitive augmentations. This study compares Polarization Sensitive OCT (PS-OCT) and Polarization Speckle measurements to better understand the relationship between polarization and coherence properties of skin. Volunteers of Fitzpatrick skin type I through VI were recruited and healthy skin was measured at four body sites (palm, inner forearm, forehead, and eye corner). Preliminary results indicate a strong similarity between the skin surface roughness measurements of PS-OCT and polarization speckle. In regards to tissue depolarization, PS-OCT measurements appear minimally affected by skin color, whereas polarization speckle was strongly affected due to differing measurement wavelengths. Among body sites, the palm and face were found to be generally smoother than the forearm; however the epidermis layer of the palm demonstrated notably greater polarization scrambling.
Polarization-Sensitive Optical Coherence Tomography (PS-OCT) is a real-time 3D imaging technique providing structural and functional contrasts in tissue. Previously, we have quantified multiple contrasts such as back-scattered intensity, accumulated phase retardation, local birefringence, and degree of polarization uniformity in the epidermis and dermis of the skin. Here, we add the Attenuation Coefficient (AC) contrast to quantify light attenuation from absorption and scattering in skin layers. Two techniques are utilized to obtain AC. One involves slope fitting to the logarithm of intensity A-lines, and the other uses a depth-resolved model-based reconstruction of AC. To investigate the effect of skin tone on AC, we first used skin phantoms with various absorption and scattering coefficients. It is found that the color of the phantom directly correlates with the absorption coefficient, while AC correlates with the sum of the absorption and reduced scattering coefficient. Darker objects, indicative of a higher concentration of melanin in the skin tissue, show a higher absorption coefficient. AC is further analyzed on in vivo skin imaging with different skin tones. However, no clear correlation between AC and skin tone is observed. This lack of correlation is likely due to the absorption coefficient being much smaller than the scattering coefficient in skin tissue at the OCT imaging wavelength of 1060nm.
Polarization-sensitive optical coherence tomography (PS-OCT) is a promising tool for non-invasive skin imaging, with its capability for depth-resolved, high-resolution, fast, and polarization-sensitive imaging. Thirty volunteers are recruited for skin imaging on the palm, arm, forehead, and eye corner. A segmentation algorithm based on intensity images will segment the epidermis and dermis layers and also stratum corneum if applicable. Multi-contrast images, including phase retardation, local birefringence, and degree of polarization uniformity, will be obtained from different skin layers. Possible relationships between the optical properties and skin features including layered structure, collagen organization, melanin concertation, and skin roughness will be investigated.
Jones matrix optical coherence tomography (JM-OCT) is a form of polarization-sensitive OCT (PS-OCT) that allows for the simultaneous and high-quality in vivo capture of multiple polarization-based imaging contrasts. Our system allows for the capture of high-sensitivity structural OCT, degree of polarization uniformity (DOPU), and birefringence images. Segmenting the epidermal-dermal junction is a topic of high interest in OCT and dermatology. While others have explored this with traditional OCT, no other groups have used this type of segmentation with JM-OCT. We believe that combining a reliable segmentation procedure with the robust and localized characterization provided by JM-OCT can help fully utilize the strengths of both techniques and allow for a better characterization of the skin layers. Here, we utilize JM-OCT to assess the skin properties of healthy volunteers. Using high-sensitivity OCT, we quantified the epidermal thickness of various locations in vivo and were able to segment the epidermis, dermis, and stratum corneum in thick skin. Polarization imaging is sensitive to specific structures in the skin, such as collagen and melanin, and we were able to quantify the depolarization, and birefringence caused by these structures in different skin layers. In thick skin, localized polarization results showed an average DOPU of 0.89 in the dermis compared to an average of 0.79 in the stratum corneum. However, in both thick and thin skin, DOPU was lower overall in the epidermis compared to the dermis, indicating that most observed depolarization occurs due to the structure of the stratum corneum. Birefringence was found to be higher in the dermis than the epidermis in both cases.
Skin cancer is one of the most prevalent types of cancer in the world, with a steadily increasing incidence rate and associated health burden [1, 2]. While it is generally treatable when detected, survival rates decrease dramatically as the disease progresses – highlighting the importance of early detection [3]. Unfortunately, the current gold standard in diagnosing skin cancers involves taking biopsies followed by histopathology, which is invasive and time consuming. Some studies have shown that the majority of biopsies ordered by primary care providers were found to be benign, meaning that biopsies are often performed when there is no cancer present [4]. Given this, there is a great interest in developing noninvasive diagnostic tools for skin imaging. Optical coherence tomography (OCT) is an imaging modality that is particularly well suited for this area, owing to its ability to provide high-resolution (3-15 μm) volumetric data at a penetration depth of up to 1.5 mm [5]. In a manner analogous to ultrasound, it provides cross-sectional images which can be comparable to histology slides [6]. However, conventional intensity-based OCT only provides structural information with no functional contrast, and as a result it has encountered difficulty in diagnosing specific cancers such as melanoma [7]. Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT which can characterize polarization properties such as the birefringence of tissue samples – birefringence specifically occurs in fibrous structures such as collagen [8]. Several groups have investigated birefringence in skin tissue using PS-OCT, but little work has been done with the more recently defined degree of polarization uniformity (DOPU) contrast. The functional contrast provided by DOPU PS-OCT imaging can provide localized, depth-resolved information on the polarization scrambling properties of samples. A specific example of this is in ophthalmic imaging, where DOPU contrast in PS-OCT imaging demonstrated the ability to segment the retinal pigment epithelium – a layer otherwise hard to differentiate in intensity-based OCT imaging [9]. To our knowledge, very few if any groups have investigated PS-OCT imaging with DOPU contrast in understanding the layered-structure of skin. We have recently investigated the DOPU in skin tissue phantoms and reported that DOPU is sensitive to surface roughness – an important factor in differentiating skin cancers from benign lesions. Our group has a previously reported PS-OCT system that can simultaneously acquire reflectance, phase retardation (birefringence), and DOPU images that we aim to use in this study to expand on our previous work and further investigate polarization properties in skin in vivo [10].
Skin cancer is the most common form of cancer in North America, and melanoma is the most deadly form of skin cancer. Roughness assessment of epidermis has been shown to be valuable in detecting potential skin neoplasia. However, the existing roughness assessment techniques cannot also provide volumetric information. For greater insight, we propose polarization sensitive optical coherence tomography (PS-OCT) for skin assessment. The intensity channel of OCT visualizes the layered structure and surface roughness profile of skin in 3D. Furthermore, PS-OCT can simultaneously conduct polarization related measurements such as the degree of polarization uniformity (DOPU) in a separate imaging channel. Skin phantoms of different surface roughness ranging from 1 to 68 μm have been studied. It was observed that for rougher surfaces, the roughness can be quantified from the surface profile visible in the intensity channel. In smoother surfaces for which the profile is not sensitive, the DOPU decreases with roughness in a quantifiable correlation. The contrast in the DOPU channel is sensitive to polarization and phase fluctuations. Smoother surfaces tend to maintain the polarization state, whereas the height differences in a rougher surface contribute to larger phase shifts between light waves within the coherence volume, leading to greater depolarization. PS-OCT was also applied to in vivo imaging of human skin. The skin at the palm edge shows lower DOPU compared to the skin on the back of the hand, an indication of greater polarization state modification caused by skin roughness. PS-OCT can provide a comprehensive evaluation of skin, which has great potential for detecting melanoma.
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